Acknowledgements
This research has been supported by grants from USDA NRI, North
American Carbon Program, NSF MRI Program, Northern Arizona
University Mission Research Program/McIntire-Stennis, Science
Foundation Arizona, and Arizona Water Institute.MULTIYEAR ANALYSIS
OF THE EFFECTS OF WILDFIRE AND THINNING ON ECOSYSTEM CARBON FLUXES
OF PONDEROSA PINE FORESTS Thomas Kolb, Sabina Dore, Mario
Montes-HeluSchool of Forestry, Northern Arizona
[email protected] ABOUT THIS
PROJECT
Amiro, B.D., A.G. Barr, et al. 2010. Ecosystem carbon fluxes
after disturbance in forests of North America. Journal of
Geophysical Research 115, G00K02, DOI 10.1029/2010JG001390.Dore,
S., T.E. Kolb, et al. 2010. Carbon and water fluxes from ponderosa
pine forests disturbed by wildfire and thinning. Ecological
Applications 20:663-683.Dore, S., T.E. Kolb, et al. 2008. Long-term
impact of a stand-replacing fire on ecosystem CO2 exchange of
ponderosa pine forests. Global Change Biology 14:120.Montes-Helu,
M., T.E. Kolb, et al. 2009. Persistent effects of fire-induced
vegetation change on energy partitioning and evapotranspiration in
ponderosa pine forests. Agricultural and Forest Meteorology
149:491-500.Roman, M.O., C.B. Schaaf, et al. 2009. The MODIS
(Collection V005) BRDF/albedo product: Assessment of spatial
representativeness over forested landscapes. Remote Sensing of
Environment 113:2476-2498.Sorensen, C.D, A.J. Finkral, et al. 2011.
Short- and long-term effects of thinning and fire on carbon stocks
in ponderosa pine stands in northern Arizona. Forest Ecology and
Management 261:460-472. Sullivan, B.W., S. Dore, et al. 2010.
Evaluation of methods for estimating soil carbon dioxide efflux
across a gradient of forest disturbance. Global Change Biology
16:2449-2460.Sullivan, B.W., S. Dore, et al. In review. Snowmelt
causes pulse emissions of carbon dioxide from snowpack at a
high-elevation site in northern Arizona, USA. Arctic, Antarctic,
and Alpine Research.Sullivan, B.W., T.E. Kolb, et al. 2008.
Thinning reduces soil carbon dioxide but not methane flux from
southwestern USA ponderosa pine forests. Forest Ecology and
Management 255:4047-4055.Sullivan, B.W., T.E. Kolb, et al. 2010.
Wildfire reduces carbon dioxide efflux and increases methane uptake
in ponderosa pine forest soils of the southwestern USA.
Biogeochemistry: DOI 10.1007/s10533-010-9499-1.Yi, C., D. Ricciuto,
et al. 2010. Climate control of terrestrial carbon exchange across
biomes and continents. Environmental Research Letters 5:03400. DOI
10.1088/1748-9326/5/3/034007.Figure 2: Relationship between vapor
pressure deficit (VPD) and light-saturated net ecosystem exchange
(NEE) at the undisturbed (UND) and thinned (THN) sites before
(AprilAugust 2006) and after (April-August 2007-9) thinning. Figure
3: Monthly light curves of mean NEE (SE) of PPFD classes for the
undisturbed (UND) and thinned (THN) sites, April-September, 2006-9.
The sites are compared for the pre-thinning period in 2006 and
after thinning in 2007-2009.Figure 1: Annual net ecosystem
production (NEP), total ecosystem respiration (TER), and gross
primary production (GPP) for the undisturbed (UND), thinned (THN),
and burned (BUR) sites in years 2006-9. Negative values represent
ecosystem sink; positive values represent ecosystem source.
WILDFIRE SITEBurned SiteThinned SiteUndisturbed SiteSITE LOCATIONS
Intensely burned in 1996 (~10,000 ha) Little tree regeneration LAI
seasonal max. 0.5 to 0.9 m2 m-2 (0% trees)Carbon stocks Burning
reduced total site C ~ 40%, 10 yrs post-fire due to a decrease tree
biomass, shift to coarse woody debris, and decrease in forest floor
Current stock ~7200 g C m-2 (46% mineral soil, 35% coarse woody
debris)
~90 ha in eddy tower footprint thinned fall 2006 after
collection of ~1 year pre-treatment data Thinning targeted small
trees and reduced tree density 70%, basal area 35%, and LAI
40%Carbon stocks Thinning reduced total site C ~19% in 1st
post-treatment year, mostly due to decrease in trees Post-thinning
stock ~ 9000 g C m-2 (38% mineral soil, 36% trees) Largest trees
logged early 1900s Natural regeneration Most mature trees ~90 yrs
old Little subsequent management
Carbon stocks Current stock ~12,400 g C m-2 (53% trees, 32%
mineral soil)
Similar soils (Eutroboralf), elevation (~2100 m), topography
(~flat)INTRODUCTIONPonderosa pine (Pinus ponderosa) forests in much
of the southwestern U.S. have become dense, mainly because of
decades of fire suppression, and are now prone to high-intensity
fire. Fuel-reduction via tree thinning is being used increasingly
to protect such forests from intense burning. To determine the
effect of both thinning and intense burning on carbon (C) dynamics
we compared ecosystem C exchange using eddy covariance for 4 years
(2006-2009) among 3 sites: Undisturbed, dense, control site with no
treatments in the last century (UND) Site subject to fuel-reduction
thinning (THN) Site subject to intense burning in 1996
(BUR)METHODSWe used the same closed-system eddy covariance approach
based on an LI-7000 IRGA, CSAT3 Campbell sonic anemometer, and CNR1
Kipp and Zonen radiation sensor at all sites. We acquired and
processed the raw data using software designed by Giovanni Manca
(Centro di Ecologia Alpina, Italy), which has been shown to provide
similar values as those produced using the Ameriflux standard
(Aubinet et al., 2000). The software applies coordinate rotation,
linear detrending, and corrects for flux losses. The fluxes are
quality flagged following steady state and integral turbulence
characteristic tests, and precipitation, scalar variances, and
spikes. Positive fluxes indicate ecosystem carbon losses; negative
fluxes carbon uptake. Missing data were gap-filled using different
methodologies, including gap-filling bad quality data or only data
with u* below the threshold of 0.2 m s-1 (Dore et al. 2008).
Variance among gap-filled estimates was used to estimate standard
errors for carbon-exchange values.CONCLUSIONS Carbon sink strength
(i.e. annual NEP) of undisturbed, dense ponderosa pine forests of
the southwestern U.S. is low relative to more mesic forests and is
sensitive to disturbance. Intense burning that converts forests to
grasslands reduces C stock (~40%) and coverts land from a C sink to
a source for decades due to low GPP when trees do not regenerate.
Fuel-reduction via thinning small trees causes a small reduction in
C stock (~19%) and a short-term reduction of C sink strength
because reduced tree leaf area reduces GPP. But, C sink recovers
rapidly in thinned stands due to increase in GPP of remaining
trees. Thinning ameliorates the impact of summer drought and high
VPD on C uptake. During drought thinned stands can be stronger
sinks than undisturbed stands.
THN
BURUNDMAIN FINDINGS
Figure 1: The burned site was a carbon source all 4 years
because TER>GPP. The undisturbed site was a C sink all years
with wide interannual variation. The thinned site had similar C
sink strength to the undisturbed site before thinning, shifted to a
weak C source the first year after thinning, and then shifted back
to a carbon sink the second and third post-thinning years. In the
usually dry year of 2009, the thinned site was a stronger sink than
the undisturbed site. Impacts of thinning on NEP were controlled
more by GPP than TER.
Figure 2: The relationship between 30-min. NEE and VPD was
similar for undisturbed and thinned sites before treatment (2006).
Thinning reduced NEE at a given VPD in the 1st post-treatment year
(2007). In the 2nd and 3rd post-treatment years (2008-9), NEE was
greater at the thinned site than the undisturbed site at high
VPD.
Figure 3: Light-response curves of 30-min. NEE vs. PPFD confirm
similar C sink strength for undisturbed and thinned sites before
treatment (2006). After treatment (2007-9), C sink was greater at
the undisturbed site vs. thinned site during wet periods, but the
opposite pattern (thinned>undisturbed) occurred during
pronounced dry periods (June 2007, July-Sept. 2009).
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